3D Scanning
Three-dimensional (3D) points in a scene can be determined using stereoscopic techniques, e.g., from stereo images acquired by two cameras. That technique depends on relating a pixel in one camera to the pixel in the other camera that corresponds to the same point in the 3D scene. However, it may be difficult to establish correspondences between the two cameras if the points are on a uniform surface. This makes it unclear exactly which of the many nearly identical pixels in one image correspond to a specific pixel in the other image.
SLS
For this reason, some 3D scanning techniques use a camera and a projector as a structured light scanner (SLS). The projector operates analogously to a camera, in the sense that 3D points in the scene can be associated with pixels on the projector image plane. Because the projector patterns are controlled by the SLS, it is easier to determine corresponding projector and camera pixels. The SLS becomes more compact as cameras and projectors decrease in size, e.g., the SLS can be arranged on a robot arm to assist automated machine operations, such as parts picking and assembly.
DLP
Many projectors use digital light projection (DLP). A DLP includes an array of digital micro mirrors (DMD) that can be switched independently to ON and OFF positions. Because the mirrors can switch rapidly, it is possible to use pulse width modulation (PWM) to project apparently intermediate light levels using different proportions of the “ON” and “OFF” times.
A pico-projector can enable a very small SLS. Pico-projectors typically use LEDs, which are not as bright as conventional projector bulbs. Lower brightness makes intensity (color) distinction difficult in many SLS applications, in particular when the scanned scene is dominated by ambient light because the ambient light makes the contrast in the structured light pattern low.
Strobing
The rapid switching ON and OFF is similar to strobing, which appears to stop motion. Strobing produces a very brief pulse of bright light.